[libc] Add support for hexagon architecture, hexagon-linux

The start.cpp is a WIP, might make sense to abstract some of the commonalities
among hexagon and x86_64?

The sin, cos, tan entrypoints are omitted to avoid a build error. I think we
might need a target-independent implementation of these?

Author: androm3da

libc/cmake/modules/LLVMLibCArchitectures.cmake

@@ -59,6 +59,8 @@ function(get_arch_and_system_from_triple triple arch_var sys_var)
     set(target_arch "riscv32")
   elseif(target_arch MATCHES "^riscv64")
     set(target_arch "riscv64")
+  elseif(target_arch MATCHES "^hexagon")
+    set(target_arch "hexagon")
   else()
     return()
   endif()
@@ -156,6 +158,8 @@ elseif(LIBC_TARGET_ARCHITECTURE STREQUAL "riscv64")
 elseif(LIBC_TARGET_ARCHITECTURE STREQUAL "riscv32")
   set(LIBC_TARGET_ARCHITECTURE_IS_RISCV32 TRUE)
   set(LIBC_TARGET_ARCHITECTURE "riscv")
+elseif(LIBC_TARGET_ARCHITECTURE STREQUAL "hexagon")
+  set(LIBC_TARGET_ARCHITECTURE_IS_HEXAGON TRUE)
 else()
   message(FATAL_ERROR
           "Unsupported libc target architecture ${LIBC_TARGET_ARCHITECTURE}")

libc/config/linux/app.h

@@ -37,11 +37,13 @@ struct TLSImage {
 
 #if defined(LIBC_TARGET_ARCH_IS_X86_64) ||                                     \
     defined(LIBC_TARGET_ARCH_IS_AARCH64) ||                                    \
-    defined(LIBC_TARGET_ARCH_IS_ANY_RISCV)
+    defined(LIBC_TARGET_ARCH_IS_ANY_RISCV) ||                                  \
+    defined(LIBC_TARGET_ARCH_IS_HEXAGON)
 // 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.
+// 64 bit for aarch64 as well.  For hexagon, argc is an int and uintptr_t
+// is 32-bits so the size will correspond.
 typedef uintptr_t ArgcType;
 
 // At the language level, argv is a char** value. However, we use uint64_t as