Switch Linux runners to Ubuntu 26.04 except for MIPS (#3703)

The i386 build ran on ubuntu-22.04, which is retired on April 17, 2027,
and ubuntu-24.04 lacks fmod and exp10 in the static i386 libm.

Build on ubuntu-26.04 for every architecture except MIPS, whose cross
toolchains Ubuntu 26.04 no longer ships, and move the decnum, oniguruma
and valgrind workflows too. Most release binaries are now built with
GCC 15 and glibc 2.43, and they remain statically linked.

.github/workflows/ci.yml

@@ -37,6 +37,8 @@ jobs:
             CC: aarch64-linux-gnu
           - arch: armel
             CC: arm-linux-gnueabi
+            # Ubuntu 26.04 has no crossbuild-essential-armel
+            packages: gcc-arm-linux-gnueabi libc6-dev-armel-cross
           - arch: armhf
             CC: arm-linux-gnueabihf
           - arch: i386
@@ -65,7 +67,8 @@ jobs:
             CC: riscv64-linux-gnu
           - arch: s390x
             CC: s390x-linux-gnu
-    runs-on: ${{ matrix.arch == 'i386' && 'ubuntu-22.04' || 'ubuntu-24.04' }}
+    # Ubuntu 26.04 no longer ships the MIPS cross toolchains.
+    runs-on: ${{ startsWith(matrix.arch, 'mips') && 'ubuntu-24.04' || 'ubuntu-26.04' }}
     env:
       AR: ${{ matrix.CC }}-ar
       CHOST: ${{ matrix.CC }}
@@ -81,7 +84,8 @@ jobs:
       - name: Install packages
         run: |
           sudo apt-get update
-          sudo apt-get install -y automake autoconf libtool crossbuild-essential-${{ matrix.arch }}
+          sudo apt-get install -y automake autoconf libtool \
+            ${{ matrix.packages || format('crossbuild-essential-{0}', matrix.arch) }}
       - name: Build
         run: |
           autoreconf -i

.github/workflows/decnum.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   disabled:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     steps:
       - name: Clone repository
         uses: actions/checkout@v7

.github/workflows/oniguruma.yml

@@ -9,7 +9,7 @@ on:
 # we test other options for --with-oniguruma here.
 jobs:
   installed:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     steps:
       - name: Clone repository
         uses: actions/checkout@v7
@@ -42,7 +42,7 @@ jobs:
             tests/*.log
 
   disabled:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     steps:
       - name: Clone repository
         uses: actions/checkout@v7

.github/workflows/valgrind.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   linux:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     steps:
       - name: Clone repository
         uses: actions/checkout@v7