etnaviv: linker: add fallback lookup to VARYING_SLOT_BFC[n]

There are valid NIR shaders pairs where the vertex shader has a VARYING_SLOT_BFC0 shader_out and the corresponding framgent shader has a VARYING_SLOT_COL0 shader_in. So at link time if there is no matching VARYING_SLOT_BFC[n], we must map VARYING_SLOT_BFC0[n] to VARYING_SLOT_COL[n].

Example shader pair from 'spec@!opengl 2.0@vertex-program-two-side back':

source_sha1: {0xf916f77d, 0xffa6ab5e, 0x160976a7, 0xb59fe59c, 0x92e8f3f6}
name: GLSL3
internal: false
stage: 0
next_stage: 4
inputs_read: 0
outputs_written: 0,13
subgroup_size: 1
bit_sizes_float: 0x20
bit_sizes_int: 0x20
first_ubo_is_default_ubo: true
flrp_lowered: true
inputs: 1
outputs: 2
uniforms: 0
decl_var shader_in INTERP_MODE_NONE vec4 gl_Vertex (VERT_ATTRIB_POS.xyzw, 0, 0)
decl_var shader_out INTERP_MODE_NONE vec4 gl_Position (VARYING_SLOT_POS.xyzw, 0, 0)
decl_var shader_out INTERP_MODE_NONE vec4 gl_BackColor (VARYING_SLOT_BFC0.xyzw, 1, 0)
decl_function main (0 params)

impl main {
        block block_0:
        /* preds: */
        vec1 32 ssa_0 = deref_var &gl_Vertex (shader_in vec4)
        vec4 32 ssa_1 = intrinsic load_deref (ssa_0) (access=0)
        vec4 32 ssa_2 = load_const (0x00000000, 0x00000000, 0x3f000000, 0x00000000) = (0.000000, 0.000000, 0.500000, 0.000000)
        vec1 32 ssa_5 = deref_var &gl_BackColor (shader_out vec4)
        vec4 32 ssa_11 = mov ssa_2
        vec4 32 ssa_13 = fsat ssa_11
        intrinsic store_deref (ssa_5, ssa_13) (wrmask=xyzw, access=0)
        vec1 32 ssa_7 = deref_var &gl_Position (shader_out vec4)
        vec4 32 ssa_12 = mov ssa_1
        intrinsic store_deref (ssa_7, ssa_12) (wrmask=xyzw, access=0)
        /* succs: block_1 */
        block block_1:
}

shader: MESA_SHADER_FRAGMENT
source_sha1: {0x5059da66, 0x00c609e5, 0x5329c39a, 0x13e2fc88, 0x8e68cb71}
name: GLSL3
internal: false
stage: 4
next_stage: 4
inputs_read: 1
outputs_written: 2
subgroup_size: 1
first_ubo_is_default_ubo: true
flrp_lowered: true
inputs: 1
outputs: 1
uniforms: 0
decl_var shader_in INTERP_MODE_NONE vec4 gl_Color (VARYING_SLOT_COL0.xyzw, 0, 0)
decl_var shader_out INTERP_MODE_NONE vec4 gl_FragColor (FRAG_RESULT_COLOR.xyzw, 0, 0)
decl_function main (0 params)

impl main {
        block block_0:
        /* preds: */
        vec1 32 ssa_0 = deref_var &gl_Color (shader_in vec4)
        vec4 32 ssa_1 = intrinsic load_deref (ssa_0) (access=0)
        vec1 32 ssa_2 = deref_var &gl_FragColor (shader_out vec4)
        intrinsic store_deref (ssa_2, ssa_1) (wrmask=xyzw, access=0)
        /* succs: block_1 */
        block block_1:
}```