xen/x86: fix usage of [[:blank:]] with BSD grep

BSD grep only recognizes [[:blank:]] as a valid expression when using
extended (modern) regular expressions.  Pass -E to the grep call used in
efi-nr-fixups.

Additionally, the return from `wc -l` is space padded on BSD, and hence
the content of efi-nr-fixups is "       2", not plain "2".  Strip the
spaces in the Makefile using $(strip ...).

Fixes: 1be65ec4c8 ('x86/EFI: avoid use of GNU ld's --disable-reloc-section when possible')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/arch.mk

@@ -100,7 +100,7 @@ endif
 ifeq ($(XEN_BUILD_PE),y)
 
 # Check if the linker produces fixups in PE by default
-efi-nr-fixups := $(shell LC_ALL=C $(OBJDUMP) -p $(efi-check).efi | grep '^[[:blank:]]*reloc[[:blank:]]*[0-9][[:blank:]].*DIR64$$' | wc -l)
+efi-nr-fixups := $(strip $(shell LC_ALL=C $(OBJDUMP) -p $(efi-check).efi | grep -E '^[[:blank:]]*reloc[[:blank:]]*[0-9][[:blank:]].*DIR64$$' | wc -l))
 
 ifeq ($(efi-nr-fixups),2)
 MKRELOC := :