testsuite: extend nopie handling to add -fno-pie to compiler flags

Some older GCC, e.g. 7.5.0 on Ubuntu 18.04 need -fno-pie to be passed to
the compiler in addition to -no-pie to be passed to the linker for non-pie
code generation.

The gdb,nopie_flag is already documented as getting passed to the
compiler, not the linker.  Use that for the new -fno-pie compiler flag and
add a new gdb,nopie_ldflag for the existing -no-pie linker flag.

CAUTION: this might break existing board files that specify
         gdb,nopie_flag.  Affected board files need to rename
         gdb,nopie_flag into gdb,nopie_ldflag.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-03-03  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* README (Note): Add nopie_ldflag.
+	* lib/gdb.exp (gdb_compile): Extend nopie handling.
+
 2021-03-02  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.ada/fixed_points.exp: Remove most special cases for minimal

gdb/testsuite/README

@@ -528,6 +528,11 @@ gdb,nopie_flag
   The flag required to force the compiler to produce non-position-independent
   executables.
 
+gdb,nopie_ldflag
+
+  The flag required to force the linker to produce non-position-independent
+  executables.
+
 gdb,debug
 
   When set gdb debug is sent to the file gdb.debug in the test output

gdb/testsuite/lib/gdb.exp

@@ -4210,16 +4210,23 @@ proc gdb_compile {source dest type options} {
 	lappend options "$flag"
     }
 
-    # Replace the "nopie" option with the appropriate linker flag to disable
+    # Replace the "nopie" option with the appropriate compiler and linker
-    # PIE executables.  There are no compiler flags for this option.
+    # flags to disable PIE executables.
     set nopie [lsearch -exact $options nopie]
     if {$nopie != -1} {
 	if [target_info exists gdb,nopie_flag] {
-	    set flag "ldflags=[target_info gdb,nopie_flag]"
+	    set flag "additional_flags=[target_info gdb,nopie_flag]"
+	} else {
+	    set flag "additional_flags=-fno-pie"
+	}
+	set options [lreplace $options $nopie $nopie $flag]
+
+	if [target_info exists gdb,nopie_ldflag] {
+	    set flag "ldflags=[target_info gdb,nopie_ldflag]"
 	} else {
 	    set flag "ldflags=-no-pie"
 	}
-	set options [lreplace $options $nopie $nopie $flag]
+	lappend options "$flag"
     }
 
     if { $type == "executable" } {