CI: update build architectures

Removed arc_archs - archs38 was marked as source-only [1].
Renamed powerpc_8540 to powerpc_8548 [2].

1. https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=c01641bcc7236d2e2de3ea65444b0cf2898df351
2. https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=2cad88b99fdae9766de84e6c1cb56f111eb53748

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>

.github/workflows/multi-arch-test-build.yml

@@ -11,10 +11,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - arch: arc_archs
-            target: archs38-generic
-            runtime_test: false
-
           - arch: arm_cortex-a9_vfpv3-d16
             target: mvebu-cortexa9
             runtime_test: false
@@ -31,7 +27,7 @@ jobs:
             target: apm821xx-nand
             runtime_test: false
 
-          - arch: powerpc_8540
+          - arch: powerpc_8548
             target: mpc85xx-p1010
             runtime_test: false