Replace sha1sum with gsha1sum on macOS

Recent macOS updates introduced /sbin/sha1sum, which is not GNU's
sha1sum and is incompatible with its implementation. This causes the
sha1sum test in make artifact to always pass because the test
currently relies on detecting the string "FAILED" in the stderr
output.

Apple's sha1sum does not support the -c (checksum verification) option,
making it unsuitable for verifying artifact integrity.

mk/common.mk

@@ -41,8 +41,14 @@ NC = \033[0m
 notice = $(PRINTF) "$(PASS_COLOR)$(strip $1)$(NO_COLOR)\n"
 
 # File utilities
-SHA1SUM = sha1sum
-SHA1SUM := $(shell which $(SHA1SUM))
+ifeq ($(UNAME_S),Darwin)
+	SHA1SUM = gsha1sum
+	SHA1SUM := $(shell which $(SHA1SUM))
+else
+	SHA1SUM = sha1sum
+	SHA1SUM := $(shell which $(SHA1SUM))
+endif
+
 ifndef SHA1SUM
     SHA1SUM = shasum
     SHA1SUM := $(shell which $(SHA1SUM))