Implement -Wpurge= (#1443)

test/asm/local-purge.err

@@ -1,3 +1,5 @@
+warning: local-purge.asm(7): [-Wpurge]
+    Purging a label ".loc"
 error: local-purge.asm(8):
     Interpolated symbol ".loc" does not exist
 error: Assembly aborted (1 error)!

test/asm/purge-multiple.err

@@ -0,0 +1,2 @@
+warning: purge-multiple.asm(9): [-Wpurge]
+    Purging an exported symbol "u"

test/asm/purge-ref.err

@@ -1,7 +1,11 @@
 error: purge-ref.asm(4):
     Symbol "ref" is referenced and thus cannot be purged
+warning: purge-ref.asm(7): [-Wpurge]
+    Purging a label "OK"
 error: purge-ref.asm(11):
     Symbol "NotOK" is referenced and thus cannot be purged
 error: purge-ref.asm(15):
     Symbol "EvenLessOK" is referenced and thus cannot be purged
+warning: purge-ref.asm(23): [-Wpurge]
+    Purging a label "Maybe"
 error: Assembly aborted (3 errors)!

test/asm/purge-refs.err

@@ -1,3 +1,5 @@
 error: purge-refs.asm(6):
     Symbol "Floating" is referenced and thus cannot be purged
+warning: purge-refs.asm(13): [-Wpurge]
+    Purging a label "Fixed"
 error: Assembly aborted (1 error)!

test/asm/purge.err

@@ -1,5 +1,9 @@
+warning: purge.asm(5): [-Wpurge]
+    Purging a label "Label"
 error: purge.asm(9):
     Symbol "Referenced" is referenced and thus cannot be purged
+warning: purge.asm(12): [-Wpurge]
+    Purging an exported symbol "Exported"
 error: purge.asm(15):
     'Undefined' not defined
 error: Assembly aborted (2 errors)!

test/asm/sym-collision.err

@@ -1,3 +1,5 @@
+warning: sym-collision.asm(20): [-Wpurge]
+    Purging a label "dork"
 error: sym-collision.asm(25):
     Interpolated symbol "dork" does not exist
 error: Assembly aborted (1 error)!

test/asm/trailing-commas.err

@@ -1,2 +1,4 @@
 warning: trailing-commas.asm(7): [-Wempty-macro-arg]
     Empty macro argument
+warning: trailing-commas.asm(22): [-Wpurge]
+    Purging a label "lobsterThermidor"