[libc][RISCV] Add naked attribute to setjmp/longjmp (#100036)

Summary:
We want to avoid any possibility that the compiler will insert a
prologue/epilogue violating the calling contracts for these special
functions, potentially clobbering registers that must be preserved. To
do that they should be marked naked, as is already the case on ARM.
See #87837 for further context.

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Differential Revision: https://phabricator.intern.facebook.com/D60251407

Author: ilovepi

libc/src/setjmp/riscv/longjmp.cpp

@@ -30,6 +30,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+[[gnu::naked]]
 LLVM_LIBC_FUNCTION(void, longjmp, (__jmp_buf * buf, int val)) {
   LOAD(ra, buf->__pc);
   LOAD(s0, buf->__regs[0]);

libc/src/setjmp/riscv/setjmp.cpp

@@ -29,6 +29,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+[[gnu::naked]]
 LLVM_LIBC_FUNCTION(int, setjmp, (__jmp_buf * buf)) {
   STORE(ra, buf->__pc);
   STORE(s0, buf->__regs[0]);