arch/_common_switches: add -latomic switch

Adding this switch saves all ab_match_arch occurrences that manually
adds '-latomic' to compiler flags.

Author: Cyanoxygen

arch/_common_switches.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 ##arch/_common_switches.sh: Switches sourced after defines.
 ##@copyright GPL-2.0+
+if ((AB_FLAGS_ATOMIC)); then CFLAGS_COMMON+=('-latomic'); fi
 if ((AB_FLAGS_SSP)); then CFLAGS_COMMON+=('-fstack-protector-strong' '--param=ssp-buffer-size=4'); fi
 if ((AB_FLAGS_SCP)); then CFLAGS_GCC_COMMON+=('-fstack-clash-protection'); fi
 if ((AB_FLAGS_RRO)); then LDFLAGS_COMMON+=('-Wl,-z,relro'); fi

lib/default-defines.sh

@@ -36,6 +36,11 @@ ABINFOCOMPRESS=1
 ABELFDEP=0	# Guess dependencies from ldd?
 ABSTRIP=1	# Should ELF be stripped off debug and unneeded symbols?
 
+# Add -latomic to compiler flags.
+# Useful when dealing with architectures lacking 64-bit and longer atomic
+# operations.
+AB_FLAGS_ATOMIC=0
+
 # Use -O3 instead?
 AB_FLAGS_O3=0