toolchain: use gcc@16.1.0 on Linux, llvm default elsewhere

A pin is still required (an unpinned package falls back to gcc-musl static,
which cannot link X11/GL), but it does not have to be llvm. Linux now builds
with gcc 16; macOS/Windows keep llvm@20.1.7. Applied to the root package and
all examples. Verified: gcc 16 builds the library, both test binaries, and all
three examples on Linux.

Author: Sunrisepeak

examples/basic/mcpp.toml

@@ -6,6 +6,7 @@ license     = "MIT"
 
 [toolchain]
 default = "llvm@20.1.7"
+linux = "gcc@16.1.0"
 
 [dependencies]
 imgui = { path = "../.." }

examples/glfw_opengl3/mcpp.toml

@@ -6,6 +6,7 @@ license     = "MIT"
 
 [toolchain]
 default = "llvm@20.1.7"
+linux = "gcc@16.1.0"
 
 [dependencies]
 imgui = { path = "../.." }

examples/minimal_window/mcpp.toml

@@ -6,6 +6,7 @@ license     = "MIT"
 
 [toolchain]
 default = "llvm@20.1.7"
+linux = "gcc@16.1.0"
 
 [dependencies]
 imgui = { path = "../.." }

mcpp.toml

@@ -11,6 +11,7 @@ path = "src/core.cppm"
 
 [toolchain]
 default = "llvm@20.1.7"
+linux = "gcc@16.1.0"
 
 [build]
 sources = [