Don't append invalid characters to symbol name

When a macro arg appears in a symbol name, the contents are appended.
However, the contents of the macro arg were not being validated.
Any character, regardless of whether it was allowed in a symbol name,
would be appended. With this change, the contents of the macro arg
are now validated character by character. The symbol name is considered
to end at the last valid character. The remainder of the macro arg is
treated as though it followed the symbol name in the asm source code.

test/asm/label-macro-arg.asm

@@ -0,0 +1,27 @@
+m1: MACRO
+x\1
+ENDM
+
+S EQUS "y"
+S2 EQUS "yy"
+
+m2: MACRO
+S\1
+ENDM
+
+	m1 = 5
+	m2 = 6
+	m1 x = 7
+	m2 2 = 8
+
+	printv x
+	printt "\n"
+
+	printv y
+	printt "\n"
+
+	printv xx
+	printt "\n"
+
+	printv yy
+	printt "\n"

test/asm/label-macro-arg.out

@@ -0,0 +1,4 @@
+$5
+$6
+$7
+$8

test/asm/label-macro-arg.out.pipe

@@ -0,0 +1,4 @@
+$5
+$6
+$7
+$8