Don't emit ELF notes on non-ELF platforms

These apparently break compilation on old MacOS toolchains, because the
MachO section name is capped to 16 chars (although, on my MacOS, at
least, the section name just gets truncated). Nevertheless, these serve
no purpose on non-ELF platforms (they're part of the LSB Linux ABI).

[Bug #20677]

coroutine/amd64/Context.S

@@ -60,6 +60,8 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 .section .note.GNU-stack,"",%progbits
 #endif
 
+#if defined(__ELF__)
+
 #if defined(__CET__) && (__CET__ & 0x01) != 0
 # define IBT_FLAG 0x01
 #else
@@ -84,3 +86,4 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 .long 0x0        /* 8-byte alignment padding */
 /* End descriptor */
 .popsection
+#endif

coroutine/arm64/Context.S

@@ -90,6 +90,7 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 #endif
 
 #if (defined(__ARM_FEATURE_BTI_DEFAULT) && __ARM_FEATURE_BTI_DEFAULT != 0) || (defined(__ARM_FEATURE_PAC_DEFAULT) && __ARM_FEATURE_PAC_DEFAULT != 0)
+#if defined(__ELF__)
 /*  See "ELF for the Arm 64-bit Architecture (AArch64)"
     https://github.com/ARM-software/abi-aa/blob/2023Q3/aaelf64/aaelf64.rst#program-property */
 #  define GNU_PROPERTY_AARCH64_FEATURE_1_BTI (1<<0)
@@ -122,3 +123,4 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
   # End descriptor
   .popsection
 #endif
+#endif