Optimize getenv performance by using self str cmp implementation

Issue: #I73MBP

Test: libc test & benchmark test
Signed-off-by: Nhaotuo <haotuo@huawei.com>
Change-Id: Ibf8667e46c5199e5b7c3039029efb5b9daacc836

musl_src.gni

@@ -2180,6 +2180,7 @@ musl_src_porting_file = [
   "src/sigchain/sigchain.c",
   "src/conf/legacy.c",
   "src/conf/sysconf.c",
+  "src/env/getenv.c",
 ]
 
 musl_inc_hook_files = [

porting/linux/user/src/env/getenv.c

+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+char *getenv(const char *name)
+{
+	if (name == NULL || __environ == NULL)
+		return 0;
+	size_t i, l = 0;
+	const char *np;
+	char **p, *ep;
+	for (; *(name + l) && *(name + l) != '='; ++l);
+	for (p = __environ; (ep = *p) != NULL; ++p) {
+		for (np = name, i = l; i && *ep; i--) {
+			if (*ep++ != *np++) {
+				break;
+			}
+		}
+		if (i == 0 && *ep++ == '=') {
+			return (ep);
+		}
+	}
+	return 0;
+}