[libc] fix -Wmissing-attributes in setjmp

Fixes:

    llvm-project/libc/src/setjmp/x86_64/setjmp.cpp:21:25: error: ‘int
    __llvm_libc_19_0_0_git::setjmp(__jmp_buf*)’ specifies less restrictive
    attribute than its target ‘int
    __llvm_libc_19_0_0_git::__setjmp_impl__(__jmp_buf*)’: ‘nothrow’
    [-Werror=missing-attributes]
       21 | LLVM_LIBC_FUNCTION(int, setjmp, (__jmp_buf * buf)) {
          |                         ^~~~~~

observed in the GCC build by manually expanding LLVM_LIBC_FUNCTION to add
`gnu::nothrow` to the alias.

We probably need to revisit adding nothrow throughout our declarations, so
there is probably a better way to clean this up in the future.

Link: llvm#88054

Author: nickdesaulniers

libc/src/setjmp/setjmp_impl.h

@@ -16,6 +16,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+[[gnu::nothrow]]
 int setjmp(jmp_buf buf);
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/setjmp/x86_64/setjmp.cpp

@@ -17,8 +17,13 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-[[gnu::naked]]
-LLVM_LIBC_FUNCTION(int, setjmp, (__jmp_buf * buf)) {
+LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::setjmp) __setjmp_impl__ __asm__("setjmp");
+
+decltype(LIBC_NAMESPACE::setjmp) setjmp [[gnu::alias("setjmp"), gnu::nothrow]];
+
+[[gnu::naked, gnu::nothrow]]
+int __setjmp_impl__ (__jmp_buf *buf) {
+
   asm(R"(
       mov %%rbx, %c[rbx](%%rdi)
       mov %%rbp, %c[rbp](%%rdi)