fix(ci): wire AMDGPU_TARGETS through backend build workflow (#9445)

Commit 8839a71 exposed AMDGPU_TARGETS as an ARG/ENV in
Dockerfile.llama-cpp so GPU targets could be overridden, but never
wired the value through the CI workflow inputs. Without it, Docker
receives AMDGPU_TARGETS="" which overrides the Makefile's ?= default,
causing all hipblas builds to compile only for gfx906 regardless of
the target list in the Makefile.

Add amdgpu-targets as a workflow_call input with the same default list
as the Makefile, and pass it as AMDGPU_TARGETS in the build-args of
both the push and PR build steps.

Assisted-by: Claude Code:claude-sonnet-4-6

Signed-off-by: Russell Sim <rsl@simopolis.xyz>

Author: russell

.github/workflows/backend.yml

@@ -30,6 +30,7 @@ jobs:
       skip-drivers: ${{ matrix.skip-drivers }}
       context: ${{ matrix.context }}
       ubuntu-version: ${{ matrix.ubuntu-version }}
+      amdgpu-targets: ${{ matrix.amdgpu-targets }}
     secrets:
       dockerUsername: ${{ secrets.DOCKERHUB_USERNAME }}
       dockerPassword: ${{ secrets.DOCKERHUB_PASSWORD }}

.github/workflows/backend_build.yml

@@ -58,6 +58,11 @@ on:
         required: false
         default: '2204'
         type: string
+      amdgpu-targets:
+        description: 'AMD GPU targets for ROCm/HIP builds'
+        required: false
+        default: 'gfx908,gfx90a,gfx942,gfx950,gfx1030,gfx1100,gfx1101,gfx1102,gfx1151,gfx1200,gfx1201'
+        type: string
     secrets:
       dockerUsername:
         required: false
@@ -214,6 +219,7 @@ jobs:
             BASE_IMAGE=${{ inputs.base-image }}
             BACKEND=${{ inputs.backend }}
             UBUNTU_VERSION=${{ inputs.ubuntu-version }}
+            AMDGPU_TARGETS=${{ inputs.amdgpu-targets }}
           context: ${{ inputs.context }}
           file: ${{ inputs.dockerfile }}
           cache-from: type=gha
@@ -235,6 +241,7 @@ jobs:
             BASE_IMAGE=${{ inputs.base-image }}
             BACKEND=${{ inputs.backend }}
             UBUNTU_VERSION=${{ inputs.ubuntu-version }}
+            AMDGPU_TARGETS=${{ inputs.amdgpu-targets }}
           context: ${{ inputs.context }}
           file: ${{ inputs.dockerfile }}
           cache-from: type=gha