ld: loongarch: Skip unsupport test cases.

ld/testsuite/ld-elf/
    * eh5.d		Skip loongarch64 target.
    * pr21884.d		Skip loongarch* targets.
    * pr26936.d		Skip loongarch* targets.

ld/testsuite/ld-elf/eh5.d

@@ -4,7 +4,7 @@
 #ld:
 #readelf: -wf
 #target: [check_as_cfi]
-#xfail: alpha-*-*ecoff hppa64-*-* tile*-*-* visium-*-*
+#xfail: alpha-*-*ecoff hppa64-*-* tile*-*-* visium-*-* loongarch64-*-*
 
 Contents of the .eh_frame section:
 

ld/testsuite/ld-elf/pr21884.d

@@ -3,7 +3,7 @@
 #ld: -T pr21884.t
 #objdump: -b binary -s
 #xfail: aarch64*-*-* arm*-*-* avr-*-* ia64-*-* m68hc1*-*-* nds32*-*-*
-#xfail: riscv*-*-* score-*-* v850-*-*
+#xfail: riscv*-*-* score-*-* v850-*-* loongarch*-*-*
 # Skip targets which can't change output format to binary.
 
 .*:     file format binary

ld/testsuite/ld-elf/pr26936.d

@@ -9,7 +9,7 @@
 # sed.  mn10300 and riscv put different numbers of local symbols in
 # linkonce section and comdat sections.  xtensa has more than one member
 # in comdat groups.
-#xfail: am33_2.0-*-* hppa*-*-hpux* mn10300-*-* riscv*-*-* xtensa*-*-*
+#xfail: am33_2.0-*-* hppa*-*-hpux* mn10300-*-* riscv*-*-* xtensa*-*-* loongarch*-*-*
 
 #...
 CU: .*/pr26936c.s: