[libc][arm] implement a basic setjmp/longjmp (#93220)

Note: our baremetal arm configuration compiles this as
`--target=arm-none-eabi`, so this code is built in -marm mode. It could be
smaller with `--target=armv7-none-eabi -mthumb`. The assembler is valid ARMv5,
or THUMB2, but not THUMB(1).

libc/include/llvm-libc-types/jmp_buf.h

@@ -32,6 +32,9 @@ typedef struct {
 #elif defined(__riscv_float_abi_single)
 #error "__jmp_buf not available for your target architecture."
 #endif
+#elif defined(__arm__)
+  // r4, r5, r6, r7, r8, r9, r10, r11, r12, lr
+  long opaque[10];
 #else
 #error "__jmp_buf not available for your target architecture."
 #endif