gcc package: skip nvptx_install for gcc~nvptx (#10672)

The nvptx installation support phase (added in 040b658) was always
running for gcc regardless of whether +nvptx was enabled. This
removes the "@when" syntax (which apparently is not working) and
adds a check inside of the "nvptx_install" function to bail out if
+nvptx is not enabled.

var/spack/repos/builtin/packages/gcc/package.py

@@ -315,12 +315,14 @@ def configure_args(self):
 
     # run configure/make/make(install) for the nvptx-none target
     # before running the host compiler phases
-    @when('+nvptx')
     @run_before('configure')
     def nvptx_install(self):
         spec = self.spec
         prefix = self.prefix
 
+        if not spec.satisfies('+nvptx'):
+            return
+
         # config.guess returns the host triple, e.g. "x86_64-pc-linux-gnu"
         guess = Executable('./config.guess')
         targetguess = guess(output=str).rstrip('\n')