test: add a test for symbol prefixes via %pragma

Add a test for symbol prefixes via %pragma, including overlapping
directives.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

test/prefixsym.asm

@@ -0,0 +1,22 @@
+; The FIRST definition of prefixes win, so more specific first
+
+
+%pragma win64  gprefix W64_
+
+%pragma win    gprefix W_
+
+%pragma elf    gprefix
+%pragma elf    lprefix .L.
+
+%pragma output gprefix _
+%pragma output lprefix L.
+
+	extern malloc
+
+	global call_malloc
+call_malloc:
+	call malloc
+	ret
+
+myfunc:
+	jmp call_malloc