[libc][arm32] define argc type and stack alignment (#96367)

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#6212stack-constraints-at-a-public-interface
mentions that the stack on ARM32 is double word aligned.

Remove confused comments around ArgcType. argc is always an int, passed on the
stack, so we need to store a pointer to it (regardless of ILP32 or LP64).

Author: nickdesaulniers

libc/config/linux/app.h

@@ -35,24 +35,6 @@ struct TLSImage {
   uintptr_t align;
 };
 
-#if defined(LIBC_TARGET_ARCH_IS_X86_64) ||                                     \
-    defined(LIBC_TARGET_ARCH_IS_AARCH64) ||                                    \
-    defined(LIBC_TARGET_ARCH_IS_ANY_RISCV)
-// At the language level, argc is an int. But we use uint64_t as the x86_64
-// ABI specifies it as an 8 byte value. Likewise, in the ARM64 ABI, arguments
-// are usually passed in registers.  x0 is a doubleword register, so this is
-// 64 bit for aarch64 as well.
-typedef uintptr_t ArgcType;
-
-// At the language level, argv is a char** value. However, we use uint64_t as
-// ABIs specify the argv vector be an |argc| long array of 8-byte values.
-typedef uintptr_t ArgVEntryType;
-
-typedef uintptr_t EnvironType;
-#else
-#error "argc and argv types are not defined for the target platform."
-#endif
-
 // Linux manpage on `proc(5)` says that the aux vector is an array of
 // unsigned long pairs.
 // (see: https://man7.org/linux/man-pages/man5/proc.5.html)
@@ -65,15 +47,15 @@ struct AuxEntry {
 };
 
 struct Args {
-  ArgcType argc;
+  uintptr_t argc;
 
   // A flexible length array would be more suitable here, but C++ doesn't have
   // flexible arrays: P1039 proposes to fix this. So, for now we just fake it.
   // Even if argc is zero, "argv[argc] shall be a null pointer"
   // (ISO C 5.1.2.2.1) so one is fine. Also, length of 1 is not really wrong as
   // |argc| is guaranteed to be atleast 1, and there is an 8-byte null entry at
   // the end of the argv array.
-  ArgVEntryType argv[1];
+  uintptr_t argv[1];
 };
 
 // Data structure which captures properties of a linux application.
@@ -87,7 +69,7 @@ struct AppProperties {
   TLSImage tls;
 
   // Environment data.
-  EnvironType *env_ptr;
+  uintptr_t *env_ptr;
 
   // Auxiliary vector data.
   AuxEntry *auxv_ptr;

libc/src/__support/threads/thread.h

@@ -43,6 +43,9 @@ union ThreadReturnValue {
      defined(LIBC_TARGET_ARCH_IS_X86_64) ||                                    \
      defined(LIBC_TARGET_ARCH_IS_ANY_RISCV))
 constexpr unsigned int STACK_ALIGNMENT = 16;
+#elif defined(LIBC_TARGET_ARCH_IS_ARM)
+// See Section 6.2.1.2 Stack constraints at a public interface of AAPCS32.
+constexpr unsigned int STACK_ALIGNMENT = 8;
 #endif
 // TODO: Provide stack alignment requirements for other architectures.
 

libc/startup/linux/do_start.cpp

@@ -69,8 +69,8 @@ static ThreadAttributes main_thread_attrib;
   // After the argv array, is a 8-byte long NULL value before the array of env
   // values. The end of the env values is marked by another 8-byte long NULL
   // value. We step over it (the "+ 1" below) to get to the env values.
-  ArgVEntryType *env_ptr = app.args->argv + app.args->argc + 1;
-  ArgVEntryType *env_end_marker = env_ptr;
+  uintptr_t *env_ptr = app.args->argv + app.args->argc + 1;
+  uintptr_t *env_end_marker = env_ptr;
   app.env_ptr = env_ptr;
   while (*env_end_marker)
     ++env_end_marker;