sha: restrict the x86 bswap asm to x86 (#41)

ByteReverse(word32) in src/sha.h uses the x86 `bswap` instruction via inline
asm, guarded only by `#if defined(__GNUC__)`. GCC and Clang define __GNUC__ on
every architecture they target, so on aarch64 the guard passes and the
assembler then rejects the instruction:

    Error: unknown mnemonic `bswap' -- `bswap %eax'

This makes the tree fail to build on any ARM64 host -- Apple Silicon, ARM
servers, a Raspberry Pi -- at a point far from anything the user did.

Narrow the guard to the architectures where the instruction exists. The
portable C fallback directly below is then used everywhere else; it was
already there and already correct.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/sha.h b/src/sha.h
index e3af7b4..ddf5cf1 100644
--- a/src/sha.h
+++ b/src/sha.h
@@ -72,7 +72,7 @@ inline word16 ByteReverse(word16 value)
 
 inline word32 ByteReverse(word32 value)
 {
-#if defined(__GNUC__)
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
     __asm__ ("bswap %0" : "=r" (value) : "0" (value));
     return value;
 #elif defined(CRYPTOPP_BYTESWAP_AVAILABLE)